A direct ab initio dynamics study of the water-assisted tautomerization of formamide

Abstract

Direct ab initio dynamics calculations based on a canonical variational transition-state theory with several multidimensional semiclassical tunneling approximations were carried out to obtain rate constants for the water-assisted tautomerization of formamide. The accuracy of the density functionals, namely, B-LYP, B3-LYP, and BH&H-LYP, were examined. We found that the BH&H-LYP method yields the most accurate transition-state properties when comparing it to ab initio MP2 and QCISD results, whereas B-LYP and B3-LYP methods predict barrier heights too low. Reaction path information was calculated at both the MP2 and nonlocal hybrid BH&H-Ž . LYP levels using the 6-31G d, p basis set. At the BH&H-LYP level, we found that the zero-point energy motion lowers the barrier to tautomerization in the formamide᎐water complex by 3.6 kcalrmol. When tunneling is considered, the activation energy at the BH&H-LYP level at 300 K is 17.1 kcalrmol. This is 3.4 kcalrmol below the zero-pointcorrected barrier and 7.0 kcalrmol below the classical barrier. Excellent agreement between BH&H-LYP and MP2 rate constants further supports the use of BH&H-LYP for rate calculations of large systems.

Computational Studies on 3-Aza-Cope Rearrangements: Protonation- Induced Switch of Mechanism in the Reaction of Vinylpropargylamine

“…The B3-LYP barrier is significantly lower, indicating that electron correlation effects are not sufficiently accounted for at this level. As already pointed out by Radom et al [65] and Bell et al, [66] this effect is frequently encountered. Comparing Walters activation energies at the MP2/6-311G* level with the LMP2/ccpVTZ values of this study one obtains a difference of 1.4 kcal mol À1 .…”

Ab Initio Study of the Prototropic Tautomerism of Cytosine and Guanine and Their Contribution to Spontaneous Point Mutations

“…Thus, a multidimensional tunneling approach is necessary here [38,39]. However, because of the high computational demand of such calculations, the latter are limited to small model systems only [40][41][42].…”

Formamide Tautomerization: Catalytic Role of Formic Acid

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Thermodynamically, the conversion of FM to FMA is endothermic, and the activation energy barrier in the gas phase is nearly 50 kcal/mol. 1,2,4,13 However, many recent investigations have predicted that the barrier is significantly reduced when the process is assisted by a water molecule 1,[2][3][4]13 or by a second formamide molecule (self-assisted process). 2,3 Investigations of such catalytic influences by a wide range of proton donor-acceptor species would be valuable to understand the mechanisms of such processes in different physical conditions.…”

“…Such information is important because many basic biochemical processes in living systems are related to tautomeric transformations analogous to that in formamide-formamidic acid system. [3][4][5][6]11,13,[17][18][19][20][21][22][23][24][25][26] In this paper, we discuss the catalytic role of formic acid on FMTFMA tautomerization in the gas phase. The 1:1 complexes of both the tautomeric forms generate doubly hydrogen-bonded cyclic structures, and in the tautomerization reaction the formic acid molecule acts as H-donor as well as H-acceptor.…”